(a) Field of the Invention
This invention relates to the field of phosphate additives and to methods of use of such additives. More particularly, the present invention is concerned with the utilization of certain inorganic phosphates, such as various calcium phosphates for the benefaction of an aggregate to improve its resistance to natural and de-icing salt induced breakdown and to improve the resistance to that breakdown of concrete and bituminous mix produced therewith. According to the invention, the inorganic phosphates are also useful for preventing spalling and breakdown of concrete surfaces due to road salt and freezing and thawing, as well as of corrosion of reinforcing steel in concrete due to road salt. The invention is also concerned with the addition of the above phosphates to road salt to reduce or prevent corrosion to cars or other vehicles. Finally, the invention is also directed to wet concrete mixtures incorporating such phosphate additives.
(b) Description of Prior Art
Some rock types, such as shales, shaley carbonates, cherts, slates, etc. are considered as deleterious when present in crushed stone, especially if these materials are intended to be used for producing concrete. If a pit or quarry contains significant percentages of these materials, for example more than 5 weight percent which is usually the case, it is considered generally unsuitable for preparing concrete. To obviate this difficulty, mechanical method of removal based on differences in gravity are now employed to separate the undesirable particles. The capital and operating costs are prohibitive to any but the largest producers.
General Electric has proposed a method of beneficiating the concrete aggregate by coating the particles with a thin polymer membrane that allows entry and egress of water into the particle but not of dissolved road salt. This treatment, like the mechanical removal mentioned above is likely to be too costly to be practical.
On the other hand it is well known that concrete is adversely affected as a result of repeated freezing and thawing and treatment with road salts. To this date, Applicant is not aware of successful treatments for the surface of concrete. Furthermore, in reinforced concrete which makes use of re-enforcing bars, the latter are rapidly subjected to corrosion mainly as a result of the penetration of de-icing salt into the substance of the concrete.
Some high alkali cements when used in concrete mix react with certain silicate and carbonate aggregates. The reaction continues over several decades, and expands and cracks the concrete. Addition of ground slag, silica fumes, and fly ash can reduce the reaction with silicate aggregates; however, no method has been found to reduce the reaction with carbonate aggregates. The proclivity of the concrete to alkali-aggregate reaction is measured by linear expansion of specimens when exposed to highly alkaline environment.
Monocalcium phosphate monohydrate was found to completely stop the reaction with both the silicate and carbonate reactive aggregates. There exist two ways to achieve this: treating the concrete after it has hardened, or treating the aggregate prior to incorporation into concrete.
Finally, there is an urgent need to treat de-icing salt in order to at least substantially reduce the enormous damages to cars and other vehicles due to corrosion.
U.S. Pat. No. 3,973,056 teaches that calcium phosphate may be used to prevent stress corrosion cracking of steel pipelines. U.S. Pat. No. 3,887,488 relies on the utilization of calcium phosphate to inhibit corrosion in sulphuric acid solutions. On the other hand, phosphates are generally known as anti-corrosive agents according to U.S. Pat. Nos. 4,000,076 and 4,168,983. U.S. Pat. No. 2,048,932 discloses a cement coating which contains sodium phosphate, and which can be used to prevent disintegration of concrete. U.S. Pat. No. 1,296,467 discloses that certain phosphates such as calcium ammonium phosphate are useful to prevent concrete from disintegrating. U.S. Pat. No. 3,087,825 mentions that compounds of phosphoric acid which form an insoluble compound with calcium hydroxide are useful to increase the corrosion resistance in cement. Finally, U.S. Pat. No. 4,054,460 discloses a cement composition which contains tricalcium phosphate. However, nothing in the prior art reveals a phosphate treatment for a concrete aggregate or method of inhibiting the corrosion effect of salt on concrete.